Ion-exchanger-based ISEs

Since the sample solutions always contain similar concentrations of an analyte ion and its counter ions, the primary requirement for Nemstian responses is selective membrane permeability for the analyte against the aqneous counter ions. The selectivity can be obtained simply by doping a membrane with ionic sites that have a charge opposite to that of the analyte. This type of membrane electrode based on ionic sites is called ion-exchanger-based ISEs or more specifically, ionophore-free ion-exchanger-based ISEs because any liquid membrane ISE has an ion-exchange capability for Nemstian responses. Table 7.1 

Membrane compositions and selectivity coefficients of ion-exchanger-based ISEs 

the equilibrium reaction can be quantified by salt-partitioning constant, Xp, as defined by 

The high membrane concentration of the analyte as a counter ion of the ionic sites shifts the equilibrium such that the aqueous counter ion is excluded from the membrane. Thus, the concentration of the aqueous anion in the cation-selective membrane doped with anionic sites is negligible in the charge balance in the membrane phase 

This selectivity sequence means that, for example, Cl in the membranes can be easily replaced with more hydrophobic anions. When it is completely replaced with another anion, the phase boundary potential becomes independent of the sample activity of CE and is determined by that of the interfering anion. The ion-exchange equilibrium is defined as 

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Among the ion-selective potentiometric sensors, the glass and the solvent polymeric membrane electrodes have become a routine tool in clinical chemistry and neurophysiological laboratories (for reviews see). Up to now, carrier and ion-exchanger based ISEs for determing the activities of HaO, Li+, Na+, K+, Ca " ", Mg ", Cl, HCO3 and CO3 in extracellular environment (blood serum, whole blood, see ) are available. Membranes selective for Li" " and suffer from interference due to Na" and Ca , respectively, but... 

Such ion-exchanger-based ISEs have been mainly used for the detection of lipophilic ions such as perchlorate and nitrate, as well as certain electrically... 

For this reason, ion-exchanger based ISEs have historically been used for the detection of perchlorate and nitrate, as well as a range of lipophilic organic ions including many dmgs and cationic/anionic surfactants. Anion exchanger-based membrane electrodes are routinely used to assess nitrate in environmental and laboratory samples. While there is some interference from bicarbonate, calibration in samples with very similar background electrolytes can minimize such effects. 

Lipophilic ionophores are essential to achieving a high sensing selectivity with liquid membrane ISEs. As explained above, ion-exchanger-based membranes always show the same selectivity pattern that follows the solvation energies of the ions, but iono-phore-based membranes may show very different selectivities. This is achieved by the formation of strong complexes between the extracted analyte ion and the ionophore in the membrane. Complex formation constants have been recently determined in the membrane phase in the range of 10 for monovalent to for divalent ions. With ion-... 

Selectivity of ion-exchanger-based ISEIs can be modified by adding an electrically neutral ionophore to the membranes (Figure 7. IB). Examples of ISEs based on commercially available neutral ionophores are summarized in Table 7.2. The neutral-ionophore-based electrodes were first developed using an antibiotic such as valinomycin as a K+-selective ionophore (K+-1). In addition to naturally occurring ionophores, many neutral iouophoies were synthesized for alkaline cations, alkaline earth cations, heavy metal ions, and inorganic anions. 

Solvent polymeric membranes conventionally consist of ionophore, ion exchanger, plasticizer, and polymer. The majority of modem polymeric ISEs are based on neutral carriers, making the ionophore the most important membrane component. Substantial research efforts have focused on the development of highly selective ionophores for a variety of analytes [3], Some of the most successful ionophores relevant to biomedical applications are depicted in Fig. 4.1. 

Consequently, an ISE for nitrate for example, a strongly hydrophilic ion, must have a strongly hydrophobic ion-exchanger ion. This conclusion has been demonstrated experimentally for a series of NO3 ISEs based on tetra-alkyl-ammonium salts with long alkyl chains [161] (see fig. 7.2). It was found that, in the studied series of substances, the tetradodecylammonium ion which is... 

An ISE for acetylcholine [11 ] is based on the same principle (ion-exchanger solution Coming No. 476200) and has been used for determination of choline... 

The solid-membrane ISE has certain disadvantages for the determination of chloride inside cells and thus ion-selective microelectrodes containing ion-exchanger Corning No. 477315 (based on a nitroxylene mixture) are used [223]. Reviews of intracellular applications of this electrode can be found in [23, 78, 86, 211,217]. 

The selectivities of Cs+ ISEs based on dibenzo-18-crown-6 derivatives <1986FZA241> or 2,3-benzoquinone 15-crown-5 ethers <1996AN127> are of limited interest, because they are not dissimilar to those found with iono-phore-free ion-exchanger ISEs < 1977ANA399>. Nowadays, the best Cs+ selectivities are obtained by using calixarene-derived crown ethers . 

Most ISEs are based on purely physicochemical and non-catalytic recognition elements solid membranes with fixed ionic sites (e.g. the glass pH electrode), ion-exchange polymer membranes or plasticised hydrogel membranes incorporating ionophores [9], Silicon oxide or metal oxides act as the recognition element in pH-ISFETs, gas-sensitive FETs, solid-state electrolyte, solid-state semiconductor and many conductometric gas sensors. 

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